Methods for Inductively-Coupled RF Power Source

ABSTRACT

A method for tracking a variable resonance condition in a plasma coil during creation of plasma from a gas flowing in a plasma torch adjacent to the plasma coil comprises: providing a radio-frequency (RF) power source comprising a power amplifier that generates a radio-frequency power signal with an adjustable operating frequency; providing a high-voltage ignition charge from said RF power source to the gas in plasma torch so as to create an electrical discharge through said gas so as to create a test sample comprising a partial plasma state within said plasma torch; and applying an RF power signal from said plasma coil to said test sample in said plasma torch, wherein said adjustable operating frequency of said power amplifier tracks said variable resonance condition of said plasma coil such that said test sample in the plasma torch achieves a full plasma state.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of and claims the priority benefit under35 U.S.C. §121 of U.S. patent application Ser. No. 12/265,870 entitled“Inductively Coupled RF Power Source,” filed Nov. 6, 2008, now U.S. Pat.No. ______, which is a continuation of and claims the priority benefitunder 35 U.S.C. §120 of U.S. patent application Ser. No. 11/285,530, nowU.S. Pat. No. 7,459,899, entitled “Inductively Coupled RF Power Source,”filed Nov. 21, 2005, the entire disclosure of which is incorporatedherein by reference.

FIELD OF THE INVENTION

The disclosed embodiments of the present invention relate generally totechniques for implementing a power source, and relate more particularlyto a system and method for implementing an inductively-coupled plasmaradio-frequency (RF) power source.

BACKGROUND OF THE INVENTION

Implementing effective methods for implementing analyticalinstrumentation is a significant consideration for designers andmanufacturers of contemporary electronic analytical devices. However,effectively performing analysis procedures with electronic devices maycreate substantial challenges for system designers. For example,enhanced demands for increased device functionality and performance mayrequire more system functionality and require additional hardwareresources. An increase in functionality or hardware requirements mayalso result in a corresponding detrimental economic impact due toincreased production costs and operational inefficiencies.

Furthermore, enhanced system capability to perform various advancedoperations may provide additional benefits to a system user, but mayalso place increased demands on the control and management of variousdevice components. For example, an enhanced electronic system thatanalyzes certain organic substances may benefit from an efficientimplementation because of the complexity and precision of the analysisinvolved.

Due to growing demands on system resources and increasing complexity ofanalysis requirements, it is apparent that developing new techniques forimplementing analytical instrumentation is a matter of concern forrelated electronic technologies. Therefore, for all the foregoingreasons, developing effective techniques for implementing analyticalinstrumentation remains a significant consideration for designers,manufacturers, and users of contemporary analytical instruments.

SUMMARY

In accordance with the present invention, a system and method aredisclosed for effectively implementing an RF power source. In oneembodiment, an RF amplifier of the RF power source provides avariable-frequency RF power signal to a fixed closely-coupled impedancematch that is implemented in a balanced manner. The impedance match thentransfers the RF power signal to a plasma coil that is positionedadjacent to a plasma torch containing a test sample for analysis. The RFpower signal is also returned through a low-pass filter to aphase-locked loop device as a reference phase signal. In addition, aphase probe is positioned near the plasma coil to sample a currentoperating frequency of the plasma coil. The output of the phase probe isreturned through a low-pass filter to the phase-locked loop as a coilphase signal.

The phase-locked loop device then employs an RF phase comparisontechnique to track a peak resonance condition at the plasma coil. Inpractice, a phase detector of the phase-locked loop device compares thereference phase signal with the sampled coil phase signal to generate anerror voltage that represents where the current operating frequency iswith respect to peak resonance. A voltage-controller oscillator of thephase-locked loop device then utilizes the error voltage to generate acorresponding RF drive signal to the RF amplifier for adjusting thefrequency of the RF power signal. The adjusted frequency of the RF powersignal operates to drive the current operating frequency of the plasmacoil in a direction towards peak resonance. At peak resonance the errorvoltage becomes zero volts.

Therefore, if the impedance at the plasma coil changes as a result of avarying load from the test sample in the plasma torch, an error voltageis produced with a polarity that drives the operating frequency of theplasma coil in a direction towards resonance. The loop response of thephase-locked loop is only tens of cycles of the operating frequency. TheRF power source may therefore rapidly track a peak resonance conditionat the plasma coil to effectively provide stable RF power and maintain aplasma state under rapid changes in load impedance. For at least theforegoing reasons, the present invention provides an improved system andmethod for effectively implementing an inductively-coupled plasma RFpower source.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the nature and objects of the invention,reference should be made to the following detailed description, taken inconjunction with the accompanying drawings, in which:

FIG. 1 is a block diagram of a plasma creation system, in accordancewith one embodiment of the present invention;

FIG. 2 is a block diagram for one embodiment of the RF power source ofFIG. 1, in accordance with the present invention;

FIG. 3 is a schematic diagram for one embodiment of the impedance matchand RF amp of FIG. 2, in accordance with the present invention;

FIG. 4 is a block diagram for one embodiment of the phase-locked loop ofFIG. 2, in accordance with the present invention;

FIG. 5 is a graph illustrating a phase shift-error voltage relationship,in accordance with one embodiment of the present invention;

FIG. 6 is a graph illustrating a technique for operating on a resonanceslope, in accordance with one embodiment of the present invention;

FIG. 7 is a flowchart of method steps for tracking a resonant conditionduring a plasma creation process, in accordance with one embodiment ofthe present invention;

FIG. 8 is a flowchart of method steps for generating an error voltage,in accordance with one embodiment of the present invention; and

FIG. 9 is a flowchart of method steps for adjusting an RF operatingfrequency, in accordance with one embodiment of the present invention.

Like reference numerals refer to corresponding parts throughout theseveral views of the drawings.

DETAILED DESCRIPTION OF EMBODIMENTS

The present invention relates to an improvement in analyticalinstrumentation techniques. The following description is presented toenable one of ordinary skill in the art to make and use the inventionand is provided in the context of a patent application and itsrequirements. Various modifications to the disclosed embodiments will bereadily apparent to those skilled in the art, and the generic principlesherein may be applied to other embodiments. Thus, the present inventionis not intended to be limited to the embodiments shown, but is to beaccorded the widest scope consistent with the principles and featuresdescribed herein.

The present invention comprises a system and method for implementing apower source, and includes a power amplifier that generates aradio-frequency power signal with an adjustable operating frequency. Thepower amplifier also generates a reference phase signal that is derivedfrom the radio-frequency power signal. An impedance match provides theradio-frequency power signal to a plasma coil that has a variableresonance condition. A phase probe is positioned adjacent to the plasmacoil to generate a coil phase signal corresponding to the adjustableoperating frequency. A phase-locked loop then generates an RF drivesignal that is based upon a phase relationship between the referencephase signal and the coil phase signal. The phase-locked loop providesthe RF drive signal to the power amplifier to control the adjustableoperating frequency, so that the adjustable operating frequency thentracks the variable resonance condition.

Referring now to FIG. 1, a block diagram of a plasma creation system 112is shown, in accordance with one embodiment of the present invention. Inthe FIG. 1 embodiment, plasma creation system 112 includes, but is notlimited to, a radio-frequency (RF) power source 116, a plasma coil 120,and a plasma torch 124. In alternate embodiments, plasma creation system112 may be implemented using components and configurations in additionto, or instead of, certain of those components and configurationsdiscussed in conjunction with the FIG. 1 embodiment.

In the FIG. 1 embodiment, plasma creation system 112 operates toinitiate and sustain a test sample in a plasma state with improved powerdelivery and efficiency characteristics. In the FIG. 1 embodiment,plasma creation system 112 may be utilized for any appropriateapplications. For example, in certain embodiments, plasma creationsystem 112 may be utilized in conjunction with Inductively-CoupledPlasma Optical Emission (ICPOE) systems or with Inductively-CoupledPlasma Mass Spectrometry (ICPMS) systems.

Plasma is known as the fourth state of matter, and is composed of anionized gas that is electrically conductive. Plasma emitselectro-magnetic waves that may be analyzed for identifyingcorresponding atomic elements in the plasma. Each element has a uniqueset of wavelengths, and the characteristics of a given wave set may beutilized to identify a corresponding element. A ratio of wavelengthintensities may be utilized to identify the concentration of eachelement in a test sample that is being analyzed. The accuracy anddynamic response of the analysis measurements depend on the stabilityand method of delivering power to initiate and sustain the test samplein a plasma state.

In the FIG. 1 embodiment, the RF power source 116 provides RF power to aplasma coil 120. A cylindrical plasma torch 124 is typically placedadjacent to the plasma coil 120. The plasma torch 124 conducts a gas,such as argon, axially through the center of the plasma coil 120. Animpedance match in RF power source 116 is employed to couple the RFpower from the RF power source 116 to the plasma coil 120 to efficientlytransfer RF power to the gas flowing through plasma torch 124.

Next, a high ignition voltage is discharged through a gas in plasmatorch 124, and the gas releases free electrons. A test sample to beanalyzed is injected into the gas stream within the plasma torch 124.The test sample is then in a conductive state to partially couple theapplied RF power from the RF power source 116. A cascade process ensuesto gradually increase the coupling and transfer of RF power from RFpower source 116 until a full plasma state is established. During theinitial ignition phase, RF power source 116 is required to supply a highlevel of RF power to initiate the cascade process towards a full plasmastate.

As a full plasma state is being established, electrical properties ofplasma coil 120 transition to a significantly different impedance. Thelower impedance reduces the RF power requirement needed to sustain theplasma. The resonance frequency of plasma coil 120 varies depending onthe particular test sample in plasma torch 124. In addition, thetransition to a full plasma state produces significant changes inelectrical properties of plasma coil 120 and plasma torch 124. RF powersource 116 must therefore effectively support the dynamics of thistransition. The implementation and functionality of RF power source 116are further discussed below in conjunction with FIGS. 2 through 9.

Referring now to FIG. 2, a block diagram for one embodiment of the FIG.1 RF power source 116 is shown, in accordance with the presentinvention. In alternate embodiments, RF power source 116 may includecomponents and configurations in addition to, or instead of, certain ofthose components and configurations discussed in conjunction with theFIG. 2 embodiment. In addition, RF power source 116 is discussed belowin the context of initiating and sustaining various types of plasma.However, in certain alternate embodiments, the principles and techniquesof the present invention may be applied to other appropriate contextsand applications.

In the FIG. 2 embodiment, RF power source 116 is implemented tofacilitate greater power stability and an extended range of operationfor plasma creation system 112 (FIG. 1). Power stability in RF powersource 116 permits more accurate measurement of various different typesof test samples. An extended range of operation facilitates analyzingcertain test samples, such as organics, that exhibit significantimpedance changes at plasma coil 120 and plasma torch 124. Increasedresponse range to impedance changes permits testing of higherconcentrations of test sample solutions.

In the FIG. 2 embodiment, an RF amplifier (RF amp) 216 provides avariable-frequency RF power signal 220 to a fixed closely-coupledimpedance match 224 that then transfers the RF power signal to plasmacoil 120 via path 228. The RF power signal 220 is also returned as anunfiltered reference phase signal 244(a) through a low-pass filter (LPF)280 to a phase-locked loop (PLL) 240 as a filtered reference phasesignal 244(b). In the FIG. 2 embodiment, LPF 280 functions to removecertain harmonic content that may be present in reference phase signal244(a). In addition, a phase probe 232 is positioned near plasma coil120 to sample the current operating state of the resonant condition atplasma coil 120. In alternate embodiments, RF power source 116 mayutilize any other appropriate techniques for sampling the resonantcondition at plasma coil 120. For example, phase probe 232 may belocated in any effective location with respect to plasma coil 120. Theoutput of phase probe 232 is returned as an unfiltered coil phase signal236(a) through a low-pass filter (LPF) 284 to phase-locked loop 240 as afiltered coil phase signal 236(b). In the FIG. 2 embodiment, LPF 284 isidentical to LPF 280 and functions to provide the same time/phase shiftas LPF 280 to maintain a ninety-degree phase relationship at resonance.

In the FIG. 2 embodiment, PLL 240 may then employ an RF phase comparisontechnique to track a peak resonance condition at plasma coil 120. Inpractice, PLL 240 compares filtered reference phase signal 244(b) withthe filtered coil phase signal 236(b) to generate an error voltage thatrepresents where the current operating frequency at plasma coil 120 iswith respect to peak resonance. PLL 240 then utilizes the error voltageto generate a corresponding RF drive signal 248 to RF amp 216 foradjusting the frequency of RF power 220. The adjusted frequency of RFpower 220 operates to drive the operating frequency of plasma coil 120in a direction towards peak resonance. At peak resonance the errorvoltage becomes zero volts.

Therefore, if the impedance at plasma coil 120 changes as a result of avarying load from plasma torch 124 (FIG. 1), an error voltage isproduced with a polarity that drives the operating frequency in adirection towards resonance. The loop response of PLL 240 is only tensof cycles of the operating frequency, which may nominally be set atapproximately 27 MHz in certain embodiments. RF power source 116therefore rapidly tracks a peak resonance condition at plasma coil 120to effectively provides stable RF power and achieve a plasma state underrapid changes in load impedance.

In the FIG. 2 embodiment, a controller 252 monitors and controls certainfunctions of RF power source 116. For example, controller 252 maymonitor various operating parameters of plasma creation system 112 (FIG.1), such as argon pressure, coolant water flow, power loss, plasmastatus, plasma door interlock, maximum current, and maximum temperature.Controller 252 may receive parameter information from any appropriatesource. For example, in the FIG. 2 embodiment, a plasma sensor 272provides plasma information to controller via path 276, and one or moretemperature sensors may provide temperature information to controller252 via path 268. If any improper operating conditions are detected,controller 252 may initiate a safe shutdown procedure. If AC power islost, controller 252 is implemented with sufficient operating power toallow controller 252 to complete the shutdown procedure. RF power source116 may bi-directionally communicate various types of relevantinformation with a host system (such as a host analytical instrument)through a host interface 264.

In the FIG. 2 embodiment, a variable power supply 260 may be utilized toselect a desired operating power for RF amp 216. The overall design ofRF power source 116 allows for an integrated compact enclosure, whereall the components, including RF amp 216, impedance match 224,controller 252, variable power supply 260, and other circuits, arehoused into one modular enclosure. This stand-alone configurationenables RF power source 116 to be incorporated into various analyticalinstruments without modification. All the components of the RF powersource 116 are housed in a common enclosure to make shieldingradio-frequency emissions more effective. The implementation andutilization of RF power source 116 is further discussed below inconjunction with FIGS. 3-9.

Referring now to FIG. 3, a schematic diagram for one embodiment of theFIG. 2 RF amp 216 and the FIG. 2 impedance match 224 is shown, inaccordance with the present invention. In alternate embodiments, RF amp216 and impedance match 224 may include components and configurations inaddition to, or instead of, certain of those components andconfigurations discussed in conjunction with the FIG. 3 embodiment.

In the FIG. 3 embodiment, a preamplifier stage (preamp) 330 of RF amp216 receives an RF drive signal 248 from PLL 240 (FIG. 2) at a givenadjustable frequency that is determined by PLL 240. Preamp 330 thenpasses the RF drive signal 248 through transformer 1 (T1) 336,transistors Q1 and Q2, and transformer 2 (T2) 324 to final stage 328 ofRF amp 216. A first transistor bank of transistors Q5, Q6, and Q7, and asecond transistor bank of transistors Q8, Q9, and Q10 are arranged in apush-pull amplifier configuration to receive the RF signal from T2 324,and generate a balanced RF power signal to impedance match 224 throughconnections 220(a) and 220(b). Impedance match 224 then passes the RFPower signal to plasma coil 120 through connections 228(a) and 228(b).In addition, the RF power output signal of RF amp 216 is sampled atconnection 220(a), and is provided in a feedback loop to PLL 240 (FIG.2) as a reference phase signal 244(a).

In the FIG. 3 embodiment, RF amp 216 has a power amplifier bias tooperate in a class E mode for improved efficiency by completelysaturating Q5, Q6, Q7, Q8, Q9, and Q10. The power amplifier may also beconfigured to operate with Q5, Q6, Q7, Q8, Q9 in a more linear orunsaturated class B mode of operation for reduced efficiency so as tosustain a plasma with a power level lower than can be achieved in thesaturated mode. This mode is advantageous for certain applications ofmass spectrometry. The design of RF amp 216 exhibits a wide bandwidthwith a flat response that delivers constant power over the range ofoperating frequencies of RF power source 116 (FIG. 2). Power amp 216 isdirectly close-coupled to impedance match 224, thus eliminating the needfor a coaxial feed cable. Close coupling permits operating at impedancesother than the characteristic impedance of a system that utilizes a 50Ohm coaxial cable. Power amp 216 may therefore operate with dynamicimpedance to allow for a greater range of operating impedances at plasmacoil 120 and plasma torch 124. Close coupling also avoids limiteddynamic range and radiation of unwanted RF often associated with coaxialcables.

Impedance match 224 is fixed (without variable components) to eliminatethe need for variable capacitors and servo systems, which are oftenslow, cumbersome, and costly. The delivery of RF power from RF amp 216through impedance match 224 to plasma coil 120 utilizes a balancedconfiguration with a grounded center tap. In alternate embodiments, anunbalanced configuration may be utilized. In the FIG. 3 embodiment, T3320 is implemented as an RF ferrite transformer that operates at animpedance of 5 Ohms or less, depending on RF power requirements. In analternate embodiment, the transformer T3 may be replaced by acenter-tapped inductor L2. This functions to de-couple the RF componentfrom the variable power supply. The load presented to the poweramplifier at 220(a) and 220(b) is 5 ohms or less, depending on RF powerrequirements. The RF power signal from RF amp 216 at connections 220(a)and 220(b) is configured to provide a balanced RF power signal intoimpedance match 224. Similarly, impedance match 224 is configured todrive plasma coil 120 in a balanced manner at connections 228(a) and228(b). The result is an RF field that is balanced around a groundpotential. Therefore, the highest voltage required in RF power source116 is reduced by one half.

Variations in the operating conditions of RF amp 216 and impedance match224 may produce unwanted resonance shifts that result in power deliveryvariations. To maintain stable operating conditions, impedance match 224is held at a constant temperature using a water cooling means. Incertain embodiments, impedance match 224 is therefore maintained at aconstant temperature to reduce changes in component values. In addition,RF power source 116 includes a heat sensor 316 that provides temperatureinformation to controller 252 of FIG. 2. In the FIG. 3 embodiment, avariable power supply 260 (FIG. 2) provides center-tapped operatingpower to impedance match 224 for powering final stage 328 of RF amp 216.In the FIG. 3 embodiment, an RF filter 332 prevents unwantedradio-frequencies from leaking into the variable power supply 260. Theutilization of RF power amp 216 and impedance match 224 is furtherdiscussed below in conjunction with FIGS. 7-9.

Referring now to FIG. 4, a block diagram for one embodiment of the FIG.2 phase-locked loop (PLL) 240 is shown, in accordance with the presentinvention. In the FIG. 4 embodiment, PLL 240 includes, but is notlimited to, a phase detector 416, an integrator 428, and avoltage-controller oscillator (VCO) 432. In alternate embodiments, PLL240 may include components and configurations in addition to, or insteadof, certain of those components and configurations discussed inconjunction with the FIG. 4 embodiment.

In order for RF power source 116 (FIG. 1) to deliver full RF power toplasma torch 124 (FIG. 1), the operating frequency of the RF power 220(FIG. 2) should preferably correspond with the natural peak resonancefrequency of impedance match 224 (FIG. 3). In the FIG. 4 embodiment, aphase-lock control feedback loop with phase detector 416 andvoltage-controlled oscillator (VCO) 432 is utilized to control the phaserelationship (and hence the frequency) between reference phase signal244(b) derived from the output of RF amp 216 (FIG. 2) and coil phasesignal 236(b) derived from the output of phase probe 232 (FIG. 2).

To track a peak resonance condition at plasma coil 120, phase detector416 must produce a zero error voltage 424. Because of certainoperational characteristics of phase detector 416, the reference phasesignal 244(b) and the coil phase signal 236(b) must be 90 degreesout-of-phase with respect to each other in order to generate a zeroerror voltage 424. If the phase relationship should differ from ninetydegrees, then the error voltage 424 from phase detector 416 would beeither positive or negative, depending on whether the phase differencewas greater or less than 90 degrees. In the FIG. 4 embodiment, referencephase signal 244(b) is derived from the RF power signal 220 that isoutput from RF amp 216, and coil phase signal 236(b) is derived from theoutput of phase probe 232, because there exists an inherent 90 degreephase shift relationship between reference phase signal 244(b) and coilphase signal 236(b) as derived from those locations.

In the FIG. 4 embodiment, error voltage 424 is provided to an integrator428 that amplifies error voltage 424 and removes any unwantedradio-frequency components in error voltage 424. Integrator 428 thenprovides the integrated error voltage 424 to VCO 432 via path 436. VCO432 responsively generates an RF drive signal 248 that has a RF drivefrequency which is determined by the amplitude and polarity of the errorsignal 424 received from integrator 428. PLL 240 then provides the RFdrive signal 248 to RF amp 216 (FIG. 2) to adjust the operatingfrequency of the RF power signal 220 that is provided to impedance match224 (FIG. 3). In the FIG. 4 embodiment, PLL 240 may be implemented toinclude a phase offset 436 that causes phase detector 416 toresponsively adjust the frequency of RF drive signal 248 so that plasmacoil 120 operates on the slope of resonance, rather than at peakresonance. For example, in certain embodiments, phase offset 436 may beimplemented by altering the length of the path of reference phase244(b). One example for operating on the slope of resonance is discussedbelow in conjunction with FIG. 6. The utilization of PLL 240 is furtherdiscussed below in conjunction with FIGS. 5-9.

Referring now to FIG. 5, a graph illustrating a phase shift-errorvoltage relationship is shown, in accordance with one embodiment of thepresent invention. The FIG. 5 graph is presented for purposes ofillustration, and in alternate embodiments, the present invention mayutilize phase shift-error voltage relationships with properties andcharacteristics in addition to, or instead of, certain of thoseproperties and characteristics discussed in conjunction with the FIG. 5embodiment.

In the FIG. 5 embodiment, phase shift values between reference phasesignals 244 and coil phase values 236 (FIG. 2) are shown on a horizontalaxis 520. In addition, error voltages 424 (FIG. 4) from phase detector416 of PLL 240 are shown on a vertical axis 516. A line 524 is plottedto represent exemplary phase-shift-error voltage relationships. Forpurposes of illustration, the FIG. 5 phase-shift-error voltagerelationship is shown as being linear. However, in alternateembodiments, various types of non-linear relationships are equallycontemplated. As discussed above in conjunction with FIG. 4, at a phaseshift of ninety degrees, a peak resonance condition 528 is shown on theFIG. 5 graph with an error voltage of zero volts. In the FIG. 5 example,as the phase shift increases above ninety degrees, the error voltageincreases, and as the phase shift decreases below ninety degrees, theerror voltage decreases.

Referring now to FIG. 6, a graph illustrating a technique for operatingon a resonance slope is shown, in accordance with one embodiment of thepresent invention. The FIG. 6 graph is presented for purposes ofillustration, and in alternate embodiments, the present invention mayoperating on a resonance slope using values and techniques in additionto, or instead of, certain of those values and techniques discussed inconjunction with the FIG. 6 embodiment.

In the FIG. 6 embodiment, operating frequency values for plasma coil 120(FIG. 2) are shown on a horizontal axis 620. In addition, amplitudes ofRF operating power at plasma coil 120 are shown on a vertical axis 616.A bell-shaped curve is plotted to represent values from exemplaryresonance conditions at plasma coil 120. In the FIG. 6 embodiment, apeak resonance condition 628 is shown at peak resonance frequency 624.RF power source 116 may be operated on the slope of resonance so thatthe operating frequency of plasma coil 120 is selectively chosen at alocation that is not directly at peak resonance 628. In the FIG. 6embodiment, RF power source 116 is being operated at slope point 636 atfrequency 632. Operating RF power source 116 on the slope of resonancemay be desirable under various types of analysis conditions, and mayprovide the ability to tailor response characteristics of RF powersource 116 for improved performance in certain operating environments.

Referring now to FIG. 7, a flowchart of method steps for tracking aresonant condition during a plasma creation process is shown, inaccordance with one embodiment of the present invention. The FIG. 7example is presented for purposes of illustration, and in alternateembodiments, the present invention may utilize steps and sequences otherthan certain of those steps and sequences discussed in conjunction withthe FIG. 7 embodiment.

In the FIG. 7 embodiment, in step 712, RF power source 116 (FIG. 2)initiates a plasma creation process by utilizing any appropriatetechniques. For example, in certain embodiments, RF power source 116 mayinitially provide a high-voltage ignition charge to a gas in plasmatorch 124 (FIG. 1). In step 714, RF power source 116 applies an RF powersignal 220 from plasma coil 120 to a test sample in a partial plasmastate in plasma torch 124. In step 716, RF power source 116 tracks aresonant condition at plasma coil 120 through a changing plasma cascadeprocess by adjusting the current operating frequency of the RF powersignal 220 provided to plasma coil 120. In step 718, the test sample inplasma torch 124 achieves a full plasma state. Finally, in step 720, RFpower source 116 maintains the full plasma state achieved in foregoingstep 718 to facilitate various analysis procedures for the test sample.

Referring now to FIG. 8, a flowchart of method steps for generating anerror voltage 424 (FIG. 4) is shown, in accordance with one embodimentof the present invention. The FIG. 8 example is presented for purposesof illustration, and, in alternate embodiments, the present inventionmay utilize steps and sequences other than certain of those steps andsequences discussed in conjunction with the FIG. 8 embodiment.

In the FIG. 8 embodiment, in step 812, RF power source 116 initiallysamples a reference phase signal 244 for generating an error voltage424. In certain embodiments, reference phase signal 244 may be derivedfrom an RF power signal 220 from RF amp 216 (FIG. 2). Then, in step 814,RF power source 116 samples a coil phase signal 236 that is generated bya phase probe 232 adjacent to plasma coil 120 (FIG. 2). In step 816, RFpower source 116 provides the reference phase signal 244 and the coilphase signal 236 to a phase detector 416 of a phase-locked loop 240(FIG. 2). Next, in step 818, phase detector 416 compares the referencephase signal 244 and the coil phase signal 236 by utilizing anyappropriate means. Finally, in step 820, phase detector 416 generateserror voltage 424 to represent the direction and the magnitude of phaseshift between reference phase signal 244 and coil phase signal 236.

Referring now to FIG. 9, a flowchart of method steps for adjusting an RFoperating frequency is shown, in accordance with one embodiment of thepresent invention. The FIG. 9 example is presented for purposes ofillustration, and, in alternate embodiments, the present invention mayutilize steps and sequences other than certain of those steps andsequences discussed in conjunction with the FIG. 9 embodiment.

In the FIG. 9 embodiment, in step 912, integrator 428 of PLL 240 (FIG.4) integrates the error voltage 424, generated by phase detector 416(FIG. 4) during step 820 of FIG. 8, to remove certain unwantedradio-frequency components. Then, in step 914, integrator 428 providesthe integrated error voltage to a voltage-controlled oscillator (VCO)432 of PLL 240. In step 916, VCO 432 generates an RF drive signal thathas a drive frequency which corresponds to the amplitude and polarity oferror voltage 424. In step 918, PLL 240 provides the RF drive signal 248to the RF power amp 216 of RF power source 116.

Finally, in step 920, in response to the drive frequency of RF drivesignal 248, RF power amp 216 generates RF power signal 220 with afrequency that tracks the current peak resonant frequency of plasma coil120. The foregoing FIG. 8 error voltage generation procedure and FIG. 9RF operating-frequency adjustment procedure are typically repeated on anongoing basis to allow RF power source 116 to track and maintainoperating parameters at current resonant conditions. For at least theforegoing reasons, the present invention provides an improved system andmethod for implementing an inductively-coupled RF power source.

The invention has been explained above with reference to certainembodiments. Other embodiments will be apparent to those skilled in theart in light of this disclosure. For example, the present invention maybe implemented using configurations and techniques other than certain ofthose configurations and techniques described in the embodiments above.Additionally, the present invention may effectively be used inconjunction with systems other than those described above. Therefore,these and other variations upon the discussed embodiments are intendedto be covered by the present invention, which is limited only by theappended claims.

1. A method for tracking a variable resonance condition in a plasma coilduring creation of plasma from a gas flowing in a plasma torch adjacentto the plasma coil, comprising: providing a radio-frequency (RF) powersource comprising: a power amplifier that generates a radio-frequencypower signal with an adjustable operating frequency, said poweramplifier also generating a reference phase signal derived from saidradio-frequency power signal; an impedance match that provides saidradio-frequency power signal to the plasma coil; a phase probepositioned adjacent to said plasma coil to generate a coil phase signalcorresponding to said adjustable operating frequency; and a phase-lockedloop device that generates a drive signal to said power amplifier tocontrol said adjustable operating frequency, said drive signal beingbased upon a phase relationship between said reference phase signal andsaid coil phase signal, such that said adjustable operating frequencytracks said variable resonance condition; providing a high-voltageignition charge from said RF power source to the gas in plasma torch soas to create an electrical discharge through said gas so as to create atest sample comprising a partial plasma state within said plasma torch;and applying an RF power signal from said plasma coil to said testsample in said plasma torch, wherein said adjustable operating frequencyof said power amplifier tracks said variable resonance condition of saidplasma coil such that said test sample in the plasma torch achieves afull plasma state.
 2. A method for tracking a variable resonancecondition in a plasma coil during creation of plasma from a gas flowingin a plasma torch adjacent to the plasma coil, comprising: providing aradio-frequency (RF) power source comprising: a power amplifier thatgenerates a radio-frequency power signal with an adjustable operatingfrequency; an impedance match that provides said radio-frequency powersignal to a plasma coil that has a variable resonance condition; aphase-locked loop device that generates a drive signal to said poweramplifier to control said adjustable operating frequency; a first powercontrol loop including a variable power supply for supplying power tothe power amplifier and a power supply controller for monitoring thepower supplied to the power amplifier and controlling the variable powersupply in response thereto to supply a desired power to the poweramplifier; and a second control loop including a phase probe positionedadjacent to said plasma coil to generate a coil phase signalcorresponding to said adjustable operating frequency; wherein the poweramplifier also generates a reference phase signal derived from saidradio-frequency power signal; and wherein the drive signal generated bythe phase-locked loop device is based upon a phase relationship betweensaid reference phase signal and said coil phase signal, such that saidadjustable operating frequency tracks said variable resonance condition;providing a high-voltage ignition charge from said RF power source tothe gas in plasma torch so as to create an electrical discharge throughsaid gas so as to create a test sample comprising a partial plasma statewithin said plasma torch; and applying an RF power signal from saidplasma coil to said test sample in said plasma torch, wherein saidadjustable operating frequency of said power amplifier tracks saidvariable resonance condition of said plasma coil such that said testsample in the plasma torch achieves a full plasma state.